Now as far as the KC705 boards, I am guessing that they will be about the same price as the ML605s, around $2000. Now, you have to watch out because the first runs of the boards are going to be ES parts (there can be bugs).
At work, we have actually already built boards (not for bitcoin of course!) using ES K7 325T devices. I haven't found any huge speed advantages over the V6 devices, so would not expect any huge frequency increases. However, the device that's going on the KC705 board is going to be larger than the ML605 (240 vs 325?), so you'll have more room for your design. However, the amount of DSP48s is about the same.
The KC705 board is now available (well, at least advertised on the Xilinx webpage) and costs $1700. It sports an XC7K325T, which has about twice the resources (logic cells, slices, flip-flops) of the Spartan6-150.
I would guess that one can put two complete miners in there, and probably run them at 300 MHz instead of 200 MHz.
In other words my best guess is that the mining performance of this board might be around 600 MH/s, give or take.
Which doesn't seem to be cost-effective, needless to say.